The present invention relates to sensing of shaft rotation and more particularly concerns methods and apparatus that enable precision sensing of shaft rotation by a low inertia device that is readily and accurately installed.
Shaft rotation sensors generally employ some type of optical, magnetic or mechanical contact arrangement to produce an electrical signal that indicates a rotational position or velocity, for example, of a rotatable shaft. Optical rotation sensing devices include both incremental and absolute rotary encoders, optical potentiometers, optical programmable switches, optical cams, optical protractors and the like, all of which embody a light source and light detector fixed relative to a rotatale shaft and a code or pattern wheel mounted for rotation with the shaft and interposed between the light source and detector so as to transmit or block light from the source to the detector in accordance with rotational position of the shaft.
Optical rotation sensors include two basic types. A more common or conventional type encoder includes its own shaft upon which the rotatable pattern wheel is mounted. A modular type is distinguished by the fact that it incoporates no shaft of its own but is adapted to be mounted directly upon the shaft of which rotation is to be sensed. The conventional encoder, which includes its own shaft upon which its pattern wheel is mounted, must incorporate bearings for rotatably mounting the shaft. The encoder shaft must be connected to the shaft of which rotation is to be sensed by some type of coupling. The encoder shaft, its bearings and the coupling all add mass to the system in addition to increasing complexity and cost of manufacture. Increased mass imposes increased inertial loads upon the system being monitored and thus introduces undesirable disturbances and decreases response times for given driving and braking forces. Further, such shaft, bearings and couplings also significantly increase the potential for error in the sensing and thus require greater care and expense in manufacture and installation or may provide output of more limited precision.
The modular encoder, having no shaft of its own, needs no bearings or additional coupling and thus may provide relatively less mass that must be rotated by the existing shaft and its drive. However, such devices are difficult and time consuming to assemble upon the shaft. Precision adjustment and rotational indexing are difficult to achieve. Assembly of such modular encoders upon the existing shaft requires removal of the instrument cover or housing and exposure of sensitive precision components. Such assembly, adjustment and alignment often require personnel with special skills and knowledge and special tools.
For example, a rotary pulse generator, which is also a shaft position encoder and digital tachometer, manufactured by Litton Systems Incorporated, employs a special aligning tool for assembly and alignment in the manner described in U.S. Pat. No. 3,900,732. In this modular device installation and alignment procedures set forth in the manufacturer's instructions include approximately 20 separate steps and generally involve initial installation of the commutator hub assembly (the pattern wheel) which is positioned by use of a special alignment tool. The next series of steps involves installation of the readout module which then must be aligned by a further series of steps involving use of an alignment tool. A final series of steps is required for establishment of the commutator air gap, which requires a special tool or gage. These procedures are accomplished with the cover removed and the sensitive precision parts exposed.
Among the many problems with the installation and alignment of prior modular encoders are the previously required employment of a relatively massive hub, having a bore of sufficient length to insure proper position of the plane of rotation of the rotating wheel assembly. Such a hub, of course, will add undesired mass thereby increasing inertia of the sensing instrument. Moreover, use of this type of hub introduces significant errors in positioning of the pattern wheel since the hub is generally secured to the shaft by means of a radially extending set screw. With conventional production tolerances a set screw can pull the center of the hub away from the center of the shaft by as much as six ten thousands of an inch which may then cause elements on the disc pattern to lead or lag nominal position by this amount. This is a significant and unacceptable magnitude of error in devices of extremely high precision.
Another disadvantage deriving from the use of the relatively large and massive hub in prior art modular devices is the difficulty of printing the desired code pattern on the assembled disc, after the code receiving portion of the disc has been assembled to the hub. Therefore, it is common to print the pattern upon the disc and, after such printing, the disc is secured to the hub itself. Even with a maximum of care and effort, unacceptable misalignment of the annular disc pattern with respect to the axis of the hub may occur.
Accordingly it is an object of the present invention to enable sensing of shaft rotation in a manner and with apparatus that avoids or minimizes above-mentioned problems.